Disclosed herein is a polymer- and etchant-free method of transferring functionalized graphene with a high degree of functional group retention. Specifically, graphene can be reductively functionalized—for example, with hydrogen—on one substrate, such as copper or silicon oxide, and delaminated from this substrate in a mixture of ethanol and water, without the use of chemical etchants. The functionalized graphene can then be reapplied to an arbitrary target substrate without use of a polymer support, and with substantial retention of functional groups. If the target substrate can withstand temperatures higher than 300° C. in a hydrogen atmosphere, the functional groups can be removed to give pristine graphene transferred without metal ion or polymer contamination. This transfer and dehydrogenation procedure can be used to create robust low-background graphene supports for use in transmission electron microscopy (TEM).
Graphene has many interesting electronic properties, but the typical method for growing graphene films is a chemical vapor deposition process on copper or nickel. The electronic properties of these highly conductive metals interfere with those of graphene. Therefore, it is generally necessary to transfer graphene from its metallic growth substrate to another substrate to take advantage of graphene's properties.
Typically, this is achieved by first coating the graphene on the growth metal with a polymer, removing the metal with a chemical etchant, transferring the graphene/polymer stack, and then removing the polymer with a suitable solvent.
The prior art transfer process leaves behind metal ion and polymer contaminants that are very difficult to remove. These impurities adversely affect many of the properties that are key to the development of graphene-specific applications. Properties affected include: electrical conductivity and charge carrier doping, surface wettability, and surface van der Waals forces. Extensive cleaning procedures have been developed to ameliorate these problems, but recent research shows that, in many cases, the contamination imparted by metal ions and polymer residue might be insurmountable. The best course of action would be to avoid these contaminants altogether.
In addition, chemical and biological sensors often operate on principles that require precise control over surface properties with minimal increase in thickness. In this case, functionalizing graphene could be a powerful strategy to independently control the properties of a substrate and its surface while adding a film only a few atoms thick.
The ideal situation would be the capability to functionalize graphene and transfer it onto an arbitrary substrate to match desired surface and substrate properties for the application at hand. However, current transfer methods of chemically functionalized graphene are typically accompanied by significant or total loss of functionality. Furthermore, some functionalization reactions on graphene are incompatible with certain substrates; in these cases, graphene must be functionalized on one substrate and then transferred to another substrate.
Thus, a transfer technique which allows retention of previously introduced chemical functionality would be an essential tool in precise surface engineering.
Here, both goals are achieved—transferring graphene without polymer or etchant, and transferring chemically modified graphene with retention of functionality—with the transfer technique described herein.